Optical system



Search Room C. G. WYNNE OPTICAL SYSTEM Feb. l2, 1946.

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Filed July 2l, 1942 thus, `QNT aN, as, Na.. Nhl, Na N2? Patented Feb.`12, 1946 Search Room OPTICAL SYSTEM Charles Gorrie Wynne, Leicester,England, assignor to Taylor, Taylor & Hobson Limited, Leicester,England, a company of Great Britain Application July 21, 1942, SerialNo. 451,755 In Great Britain July 22, 1941 20 Claims.

This invention relates to an optical system, intended more especiallyfor use in what may be termed shadow projection, that is in projectlngan image of the profile of a solid object illuminated by collimatedlight from a source on the side of the object remote from the system,the optical system being of the kind corrected for spherical andchromatic aberration (including oblique colour), coma, astigmatism,field curvature and distortion, and comprising two convergent members ofwhich the front member consists of three separated components, namely adivergent component located between tyvo convergent components, whilstthe rear member constitutes a field member and may consist of a singlecompound component, or of two or more components with only relativelysmall air separations, the axial separation between the rear surface ofthe front member and the front surface of the eld member being greaterthan half the equivalent focal length of the whole system. It should bemade clear that the front of the system is the side of the longerconjugate in accordance with the usual convention (so that when thesystem is used for profile projection purposes the light will passthrough the system from the rear to the front). The field member isadapted, when the system is used for prole projection, to form an imageof th'e light source at the optical stop.

It is to be understood that the term component is used herein toindicate a part of the system (whether consisting of a. single lenselement or compounded of two or more lens elements) separated bysubstantial air gaps from the other parts of the system. It is furtherto be understood that the terms compounded" and contact surface are usedherein Whether or not the two cooperating surfaces together constitutingthe contact surface between each pair of elements have exactly the samecurvature and whether or not cement is actually employed to connect thesurfaces with one another.

British patent specification No. 444,350 gives examples of an opticalsystem of the above-mem tioned kind, which satisfactorily effect profileprojection so long as the incident collimated beam is substantiallyparallel to the optical axis of the system.

The present invention has for its object to pro vide an improved opticalsystem of the above kind which will be suitable inter alla for theprojection of the profile of an axial section of a screwthread, whereinit is desirable for the optical axis of the system to be at right anglesto the thread axis and for the illuminating beam to be inclined at themean helix angle to such optical axis.

To attain this object according to the invention the radius of curvatureof the front surface of the rear convergent component of the frontmember should lie between .75 and 3.0 times the equivalent focal lengthof the whole system. If profile projection at high magnification isrequired, it is preferable for such radius t0 be less than 1.75 timesthe equivalent focal length, and also for the power of a collectivecontact surface in the field member to be greater than .25 times theequivalent power of the system. It is to be understood that the termpower herein used refers to the optical power, that is the reciprocal ofthe focal length. The two members are preferably separatelyachromatised.

The two members of the system are preferably so arranged that the exitpupil of the system, when used for profile projection, lies within thefront member or only a short distance outside such member. Whencollimated light is used to illuminate the object whose profile is to beprojected, the objective will form an image of the t light source at theexit pupil, and if the source itself is of suitable small size, suchimage will act as a stop. In practice, however, it will usually bepreferable to provide an actual stop in this position, and when theobject under examination is a screwthread and the illuminating device isinclined at the appropriate angle to the optical axis to suit the helixangle of the thread, the stop is preferably elongated in the planecontaining the optical axes of the system and of the illuminatingdevice, to allow for the increased displacement of the image of thelight source in such plane due to the inclination of the beam.

Another object of the invention is to provide a complete proileprojection apparatus for the examination of screwthreads, by projectingan image of the profile of an axial section of the thread.

Further objects of the invention will be apparent from the appendedclaims and from the following description of the accompanying drawing,in which Figure 1 shows diagrammatically 9, complete profile projectionapparatus according to the invention,

Figure 1a is a detail view of a part thereof,

Figures 2 and 3 show alternative forms of optical system for use in theapparatus shown. in Figure 1, and

Figures 4 to 7 show further variants of the optical system, those ofFigures 4 and 6 being illustrated in their operative positions in acomplete projection apparatus.

In the construction of Figures 1 and 2, the projection apparatuscomprises a convergent field member A and a convergent front member B,together constituting the optical system, the iield member A consistingof a cemented doublet formed of a convergent element in front of adivergent element, whilst the front member B consists of a separatedtriplet formed of a divergent element located between two convergentelements.

The object, whose profile is to be examined, is disposed behind theoptical system in the object plane indicated at C, and is illuminated bya collimated beam of light projected from a lamp filament D of smallsize by means of a collimator E. Although the apparatus can be employedgenerally for profile projection, it is more especially intended forprojecting an image of the profile of an axial section of a screwthread,and the aD- paratus is illustrated in the drawing with reference to itsuse for this purpose.

The screwthread under examination is mounted in a suitable support withits axis lying in the object plane C, and the illuminating device,comprising collimator and lamp filament, is disposed with its opticalaxis inclined to the axis of the optical system AB at an angle equal tothe mean helix angle of the screwthread, such helix angle of coursevarying from the crest to the hollow of the thread. Owing to the factthat the lamp filament D, though small, is of finite size, the rays fromthe collimator E will not all be parallel to its axis and in fact abundle of parallel rays at a slight angle to such axis will emerge fromthe collimator from every point of the filament, the angle depending onthe distance of the point from the axis of the collimator. The size ofthe filament is so chosen that such angular deviation will cover thedifference between the helix angles at the crest and in the hollow ofthe thread. For example the illuminating device may be inclined at anangle of 6 to the optical axis of the system AB to suit the mean helixangle of the thread, and the size of the filament may be such as to givean angular spread on either side of the axis of 2 to cover thevariations or the helix angle. 'I'he extreme rays in the planecontaining the axes of the illuminating device, the screwthread and theoptical system will thus be inclined at 8 to the optical axis of' thesystem, whilst the extreme rays in the plane at right angle to suchplane will be inclined at 2 to the optical axis. The image of thefilament produced at the exit pupil of the optical system will thus beelongated, and a stop F with an aperture of the appropriate shape shownin Figure 2, is provided at such exit pupil to suit such image. Thussuch stop aperture may have a breadth F/ 14 corresponding to the 2 and alength F/3.6 corresponding to the 8. In practice, the stop apertureshould be made slightly larger than the filament image to avoiddiffraction diiliculties.

In order to be applicable to the examination of screwthreads ofdifferent helix angles, the illuminating device is preferably pivotedabout a point on the optical axis of' the system AB, so that it can beadjusted to any desired small inclination. The elongated stop F may beremovable to permit substitution of other stops of appropriate apertureshapes to suit different inclinations of the illuminating device oralternatively the stop may consist of a pair of overlapping slottedleaves mutually adjustable to vary the length of the stop aperture asrequired. The actual position of the pivot for the illuminating deviceon the axis of the optical system is not important, provided that thecollimator lens is large enough to illuminate the object throughout therange of angular adjustment, but it will commonly be convenient for suchpivot axis to lie in or close to the object plane of the system. It willbe clear that with this arrangement an image of the profile of an axialsection of the screwthread will be projected by the optical system on toa projection screen in the image plane G of the optical system. Theoptical system may be designed to give a high magnification. say X50, ona distant screen, as in the construction shown in Figure 1, oralternatively a lower magnification on a nearer screen.

The arrangement of the optical system may vary to suit differentrequirements, and numerical data for a number of convenient practicalex- 40 amples are given in the following tables, the first examplecorresponding to that shown in Figure l, whilst the remaining examplesare illustrated in the correspondingly numbered figures of the drawing.In these tables R1, Rz represent the radii of curvature of theindividual surfaces counting from the front (the positive signindicating that the surface is convex to the front and the negative signthat it is concave thereto), D1, Dz represent the axial thicknesses oithe individual elements, and Si, Sz. Sa represent the axial airseparations between the components. 'I'he tables also give the meanrefractive index nn (for the D-line) and the Abb V number for the glassused for each element.

Example III [Equivalent focal length 1.000, relative aperture F/l6]Thickness Refractive Abb V Radius segrglon index np number Dr 0499 1.6142 55. 12x-2. 762

Si 2146 Rx. 4631 S: 1582 Bri-1. 1856 D: 0499 1. 6443 48. 4 Rr'. 4167 Si6612 R14-2 1923 DI 0845 1. 574 59. 4 R|. 2930 D; 0190 1. 652 33. 5 Rl-l.131

The above examples are all intended for profile projection at highmagnification (X50) using collimated light inclined to the axis ofprojection and employ a doublet for the field member. The radius R5 isin each case `between .75 and 1.75 and the power of the collectivecemented surface Re in the field member is .284 in Example I, .277 inExample II and .266 in Example III. Examples I and II differ from oneanother primarily in that the field member in Example II has itsdivergent element in front of its convergent element, whilst in ExampleI the convergent element is in front of the divergent element. ExampleIII differs from Example I mainly in the -use of different glasses inthe field member. In Examples I and III the stop F is just outside thefront member at .017 in iront of the front surface R1, whilst in ExampleII the stop is within the front member at .126 in front of the frontsurface R: of the divergent element. 'I'he distances from the frontsurfaces R1 to the image plane G and from the rear surface Re to theobject plane C are respectively 49.0 and .489 in Example I, 49.8 and.473 in Example II and 50.0 and .498 in Example III.

In the fourth example, which is shown in Figure 4 in a completeprojection apparatus, the field member is divided into two and consistsof a convergent doublet in front of a separate convergent element. Thisexample, which is likewise corrected for high magnification (X50) and lssuitable for proiile projection of a screwthread or the A Search Room byway of example with its illuminating device in the axial position ofadjustment), has its radius Re 1.18 whilst the power of the collectivecemented surface Rs is .268. In this example the stop is positioned at.017 in front of the surface R1, the image plane is distant 50.0 infront of such surface and the object plane is .475 behind the rearsurface Rn.

Example IV [Equivalent local length 1.000, relative aperture F/l]Thickness Refractive Abb V Radius 1.25 index up number Di 0499 1. 014255. 5 Iii-2. 758

Si 2142 R|-. 4624 Si 1530 Bri-1. 184

D: 0499 1. 6443 48. 4 Rc. 4161 Sl 6002 R14-36. 810

DI 1043 1. 613 55. 7 R|-. 3146 D5 0189 l. 6973 30. 5 Rl-l. 399

Si 0 R1n+1. 995

De 0399 1. 613 55. 7 lin-13. 836

The following two examples differ from the foregoing examples, in thatthey are corrected4 for a low magnification (X3) and are suitable foruse in microsope investigation 0f the profile of a screwthread or thelike illuminated by collimated light inclined to the axis of projection.Both employ a doublet for the field member, the convergent element beingin front of the divergent element in Example V and behind the divergentelement in Example VI. These examples each have a relative large fifthradius. greater than 1.75 but less than 3.0, and the convergent power ofthe cemented surface Re is .283 in Example V but only .140 in ExampleVI. The stop position in each of these examples is within the frontmember at .075 in Example V and .004 in Example V'I in front of thethird surface Ra. In Example V the image plane is 2.694 in front of thefront surface R1 and the object plane is .693 behind the rear surfaceRn, the corresponding iigures for Example VI being 2.368 and .654respectively.

It is not essential to the invention for the three components of thefront member to consist of simple elements, and one or more of them maybe made compound, if desired. One such arrangement is shown in Figure 7with numerical data as in the following table.

Example VII [Equivalent focal length 1.000, relative aperture F/16]Thickness Refractive Abb V Radius serglon index ma number D1 0244 1.6142 55. 5 .RH-2. 049

D: 0244 1. 584 50. 2 Ita-2. 412

S1 2101 Ri. 4535 S1 1550 RVi-1. 1612 D4 0489 l 6443 48 4 R1 4081 S3 6477Rari-2. 2212 Da 0816 1. 0973 30. 5 Rin-1. 1800 In this example theposition of the stop F, the distance from the front surface R1 to theimage plane, and the distance from the rear surface R to the objectplane, are substantially the same as the corresponding dimensions givenabove for Figure 1.

It will be appreciated that although the arrangements have beendescribed more especially with reference to their use in examiningscrewthreads, they can also be usefully employed for other profileprojection purposes.

What I claim as my invention and desire to secure by Letters Patent is:

l. An optical system corrected for spherical and chromatic aberrations,coma, astigmatism, field curvature and distortion, and comprising twoconvergent members in axial alignment, the front member consisting of adivergent component located between two convergent components whilst therear member constitutes a field member and consists of at least twoelements, the axial separation between the rear surface of the frontmember and the front surface of the field member being -greater lthanhalf the equivalent focal length of the whole system, whilst the radiusof the front surface of the rear convergent component of the frontmember lies between .75 and 3.0 times the equivalent focal length of thewhole system.

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2. An optical system as claimed in claim 1, in which the radius ofcurvature of the front surface of the rear convergent component of thefront member is less than 1.75 times the equivalent focal length of thewhole system.

3. An optical system as claimed in claim 1, in which the field memberincludes a Collective contact surface whose power is greater than .25times the equivalent power of the whole system, and in which the radiusof curvature of the front surface of the rear convergent component ofthe front member is less than 1.75 times the equivalent focal length ofthe whole system.

4. An optical system as claimed in claim 1, in which the two members areseparately achromatised.

5. An optical system corrected for spherical and chromatic aberrations,coma, astigmatism, field curvature and distortion, and comprising twoseparately achromatised convergent members in axial alignment, the frontmember consisting of a divergent component located between twoconvergent components whilst the rear member constitutes a field memberand consists of a convergent element compounded with a divergentelement, the axial separation between the rear surface of the frontmember and the front surface of the field member being greater than halfthe equivalent focal length of the whole system, whilst the radius ofcurvature of the front surface of the rear convergent component of thefront member lies between .75 and 3.0 times the equivalent focal lengthof the whole system.

6. An optical system corrected for spherical and chromatic aberrations,coma, astigmatism, eld curvature and distortion, and comprising twoseparately achromatised convergent members in axial alignment, the frontmember consisting of a divergent component located between twoconvergent components whilst the rear member'constitutes a field memberand includes a compound component having a collective contact surfacewhose power is greater than .25 times the equivaient power of the wholesystem, the radius of curvature of the front surface of the rearconvergent component of the front member lying between .75 and 1.75times the equivalent focal length of the whole system.

7. An optical system corrected for spherical and chromatic aberrations,coma, astigmatism, field curvature and distortion and comprising twoconvergent members in axial alignment, the front, member consisting of adivergent component located between two convergent components whilst therear member constitutes a field member and consists of a convergentelement cemented in front of a divergent element the power of thecemented surface being greater than .25 times the equivalent power ofthe whole system, the axial separation between the rear surface of thefront member and the front surface of the field member being greaterthan half the equivalent focal length of the whole systemI whilst theradius of curvature of the front surface of the rear convergentcomponent of the front member lies between .75 and 3.0 times theequivalent focal length of the whole system.

8. An optical system corrected for spherical and chromatic aberrations,coma, astigmatism, field curvature and distortion, and comprising twoconvergent members in axial alignment, the front member consisting of adivergent component located between two convergent components whilst therear member constitutes a field member and consists of a convergentelement compounded 'nSuC with and disposed in front of a divergentelement, the radius of curvature of the front surface of the rearconvergent component of the front member lying between .75 and 1.75times the equivalent focal length of the whole system.

9. An optical system corrected for spherical and chromatic aberrations,coma, astigmatism, field curvature and distortion, and comprising twoconvergent members in axial alignment, the front member consisting of adivergent component located between two convergent components whilst therear member constitutes a field member and consists of a convergentelement compounded and disposed behind a divergent element, the axialseparation between the rear surface of the front member and the frontsurface of the field member being greater than half the equivalent focallength of the whole system, whilst the radius of curvature of the frontsurface of the rear convergent component of the front member liesbetween .75 and 3.0 times the equivalent focal length of the wholesystem.

10. An optical system corrected for spherical and chromatic aberrations,coma, astigmatism, fleld curvature and distortion, and comprising twoconvergent members in axial alignment, the front member consisting of adivergent component located between two convergent components whilst therear member constitutes a field member and conlet diso ne. 3 separatecomergenmgngit) the axial separation e ween the rear surface of thefront member and the front surface of the field member being greater'than half the equivalent focal length of the whole system, whilst theradius of curvature of the front surface of the rear convergentcomponent of the front member lies between .75 and 3.0 times theequivalent focal length of the whole system.

11. An optical system corrected for spherical 40 and chromaticaberrations, coma, astigmatism, eld curvature and distortion, andcomprising two convergent members in axial alignment, the front memberconsisting of a divergent component located between two convergentcomponents whilst the rear member constitutes a field member andconsists of ouble close to a separate convergent element the contac surace ou e av ng collective power greater than .25 times the equivalentpower of the whole system, whilst the radius of curvature of the frontsurface of the rear convergent component of the front member liesbetween .75 and 1.75 times the equivalent focal length of the wholesystem.

12. An optical system as claimed in claim 8, having numerical datasubstantially as set forth in the following table:

[Equivalent focal length 1.000, relative aperture F116] 0 wherein R1, R2

Search Room wherein R1, Rn represent the radii of curvature of theindividual surfaces counting from the front (the positive signindicating that the surface is convex to the front and the negative signthat it is concave thereto), D1, D2 represent the axial thicknesses ofthe individual elements, and Si, Sz, Sa represent the axial airseparations between the components.

13. An optical system as claimed in claim 9. having numerical datasubstantially as set forth in the following table:

[Equivalent focal length 1.000, relative aperture F/l] ThicknessRefractive Abb V nms selgrgon index 'np number Rr'r. 5864 Di 0801 1.0125 59. 3 Rz-l. 789

DI 0250 1.6212 36. 1 Rrr. 2903 S: 1802 [fri-2. 659

DI 0882 1. 6125 59. 3 Rs-n 3318 Ss 8342 R7+1. 644

Ds 0952 1. 6125 55. 7 .Rl-1. 300

represent the radii of curvature of the individual surfaces countingfrom the front (the positive sign indicating that the surface is convexto the front and the negative sign that it is concave thereto), D1, D2represent the axial thicknesses of the individual elements, and Si, S2,Sa represent the axial air separations between the components.

14. An optical system as claimed in claim 10, having numerical datasubstantially as set forth in the following table:

[Equivalent focal length 1.000, relative aperture F/ll ThicknessRefractive Abb V Radms seg-fion index up number RFI'. 78%

Di 0499 1. 6142 55. 5 Rx-Z. 758

Sl 2142 Ra. 4624 S1 1580 Rri-l. 184

D: 0499 l. 6443 48. 4 R|. 4161 Si 6602 R14-36. 810

D4 1043 1. 613 55. 7 Rg 3146 Dl .0189 1. 6973 30. 5 Rr-L 399 S4 0Rn-i-l. 995

De 0399 1. 613 55. 7 R11-13. 835

wherein R1, Rz represent the radii of curvature of the individualsurfaces counting from the front (the positive sign indicating that thesurface is convex to the front and the negative sign that it is concavethereto), D1, D: represent the axial thicknesses of the individualelements, and S1. Sz, S3 represent the axial air sep arations betweenthe components.

15. In optical apparatus for projecting an image of the profile of anobject, the combination with the optical system claimed in claim 1, of asource of light of small size and a. collimator for directing acollimated beam of light derived from whereby an image of the protlle ofthe object will be projected by the system.

16. In optical apparatus for projecting an image of the profile of anobject, the combinationV with the optical system claimed in claim 6, ofaV source of light of small size, a collimator for directing acollimated beam of light derived from such source past the object to theoptical system, a small stop located at the exit pupil of the systemclose in front of the front member thereof, and a projection screen forreceiving the image of the profile of the object projected by thesystem.

17. In optical apparatus for projecting an image of the profile of anobject, the combination with the optical system claimed in claim 5, of asource of light of small size, a collimator for directing a collimatedbeam of light derived from such source past the object to the opticalsystem, a. small stop located at the exit pupil of the system, such exitpupil lying within the front member of the system, whereby an image ofthe profile of the object will be projected by the system.

18. In optical apparatus for projecting an image of the profile o1' anaxial section of a screwthread, the combination with the optical systemclaimed in claim 1, of an illuminating device disposed with its axisinclined to the m axis of the system at an angle appropriate vto the`helix angle of the screwthread and comprising a source of light of smallsize and a collimator for directing a collimated beam of light derivedfrom the source past the screwthread to the optical system, whereby animage of the proille will be projected by the system.

19. In optical apparatus for projecting an image of the proille of anaxial section of a screwthread, the combination with the optical systemclaimed in claim 1, of an illuminating device disposed with its axisinclined to the optical axis of the system at an angle appropriate tothe helix angle of the screwthread and comprising a source of light ofsmall size and a collimator for directing a. collimated beam ot lightderived fromv the source past the screwthread to the optical system, anda small stop located at the exit pupil of the system and elongated inthe plane containing the optical axes of the system and of theilluminating device to suit the inclination of such device, whereby animage oi' the prole will be projected by the system.

20. In optical apparatus for projecting an image of the profile of anaxial section of a screwthread, the combination with the optical systemclaimed in claim 1, of an illuminating device mounted to pivot about apoint on the optical axis of the system so that its inclination to suchaxis can be adjusted to suit the helix angle of the screwthread, suchdevice comprising a source of light of small size and a collimator fordirecting a collimated beam of light derived from such source past thescrewthread to the optical system, whereby an image of the profile willbe projected by the system.

CHARLES GORRIE WYNNE.

